1. Field of the Invention
The invention pertains to powder metallurgical compositions and methods for preparing rare earth-iron-boron permanent magnets, and to magnets prepared by such methods.
2. Description of the Art
Permanent magnets (those materials which exhibit permanent ferromagnetism) have, over the years, become very common, useful industrial materials. Applications for these magnets are numerous, ranging from audio loudspeakers to electric motors, generators, meters, and scientific apparatus of many types. Research in the field has typically been directed toward developing permanent magnet materials having ever-increasing strengths, particularly in recent times, when miniaturization has become desirable for computer equipment and many other devices.
The more recently developed, commercially successful permanent magnets are produced by powder metallurgy sintering techniques, from alloys of rare earth metals and ferromagnetic metals. The most popular alloy is one containing samarium and cobalt, and having an empirical formula SmCo.sub.5. Such magnets also normally contain small amounts of other samarium-cobalt alloys, to assist in fabrication (particularly sintering) of the desired shapes.
Samarium-cobalt magnets, however, are quite expensive, due to the relative scarcity of both alloying elements. This factor has limited the usefulness of the magnets in large volume applications such as electric motors, and has encouraged research to develop permanent magnet materials which utilize the more abundant rare earth metals, which generally have lower atomic numbers, and less expensive ferromagnetic metals. The research has led to very promising compositions which contain neodymium, iron, and boron in various proportions. Progress, and some predictions for future utilities, are given for compositions described as R.sub.2 Fe.sub.14 B (where R is a light rare earth) by A. L. Robinson, "Powerful New Magnet Material Found," Science, Vol. 223, pages 920-922 (1984).
Certain of the compositions have been described by M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, and Y. Matsuura "New Material for Permanent Magnets on a Base of Nd and Fe," Journal of Applied Physics, Vol. 55, pages 2083-2087 (1984). In this paper, crystallographic and magnetic properties are reported for various Nd.sub.x B.sub.y Fe.sub.100-x-y compositions, and a procedure for preparing permanent magnets from powdered Nd.sub.15 B.sub.8 Fe.sub.77 is described. The paper discusses the impairment of magnetic properties which is observed at elevated temperatures and suggests that additions of small amounts of cobalt to the alloys can be beneficial in avoiding this impairment.
Additional information about the compositions is provided by M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, and K. Hiraga, "Permanent Magnet Materials Based on the Rare Earth-Iron-Boron Tetragonal Compounds," IEEE Transactions on Magnetics, Vol. MAG-20, Sept. 1984, pages 1584-1589. Small additions of terbium or dysprosium are said to increase the coercivity of neodymium-iron-boron magnets; a comparison is made between Nd.sub.15 Fe.sub.77 B.sub.8 and Nd.sub.13.5 Dy.sub.1.5 Fe.sub.77 B.sub.8 magnets